An internal combustion engine (ICE) commonly employs a pressurized cooling system with a circulating liquid coolant for cooling the engine. Waste heat is transferred from the ICE to the coolant in a cooling jacket(s) surrounding combustion heated parts of the engine. The heat absorbed by the circulated coolant is generally dissipated by a heat exchanger into the air. This heat exchanger, also known as a “radiator”, may also operate with a cooling fan which blows air into the heat exchanger thereby promoting heat transfer from liquid coolant to air.
Scaling Considerations for Engine Cooling Systems: The design capacity of ICE cooling system is traditionally determined according to the cooling capacity needed for the most severe operating conditions of the particular ICE installation such as conditions of high engine output, low vehicle speed, and/or hot ambient temperatures. Heat transfer capacity of the radiator is dependent on the temperature of ambient air. In particular, in cool temperatures, the radiator may be capable of transferring substantially more heat to ambient air than in hot ambient conditions. If the engine is used in automotive vehicle, higher speed of the vehicle generates more favorable conditions for increased heat transfer by the radiator. Normally, coolant circulation between the engine and the radiator is controlled by a temperature control valve (such as a thermostatic valve). The temperature control valve regulates the coolant flow so that the coolant temperature is maintained near a predetermined “normal” operating temperature. However, under heavy load and/or during high ambient temperature conditions, the rate at which waste heat is transferred from the engine into the coolant may exceed the capacity of the radiator to transfer such heat to ambient air. As a result, the coolant temperature may rise above the predetermined normal operating temperature. If the heat load is not reduced, coolant temperature may approach the coolant boiling point, a coolant pressure relief valve may open, and substantial loss of coolant from the system may occur.
To prevent frequent thermal overload, the heat load handling capacity of a given-size ICE cooling system may be increased by using one of the two principal approaches: 1) increasing the system's physical size or 2) increasing the system's operating temperature. Increasing the physical size of the cooling system may be accomplished, for example, by increasing the size of the radiator core, capacity of the coolant pump (also known as water pump), capacity of the cooling fan, or some combination of these. In automotive applications, however, space in the engine compartment is becoming very scarce in part due to downsizing of vehicle engine and body motivated by the desire to increase fuel economy and reduce harmful emissions. In particular, downsized engines often require a supercharger and a charge air cooler to attain acceptable acceleration. Such equipment requires significant volume in the engine compartment. In addition, increasing the volume of cooling fluid in the system negatively impacts warm-up characteristics of the engine, which translates to increased cold start emissions. Furthermore, increasing the capacity of the water pump and/or cooling fan also increases parasitic losses and reduces the overall engine system efficiency.
Increasing the operating temperature of the cooling system is a well-known approach to increasing thermal handling capacity of the system without increasing its physical size. With higher temperature difference between coolant and ambient air at the radiator core, heat dissipation capacity of the radiator is significantly increased. Operating temperature of the cooling system is also related to its operating pressure, which should held at a sufficiently high level to prevent the coolant from boiling. In particular, the operating temperature of many cooling systems for automotive engines in current production is about 100 degrees Centigrade (215 degrees Fahrenheit). In these systems, a pressure relief valve is typically set to open at about 15 psig, which is the vapor pressure of water-based coolant corresponding to a coolant temperature of about 120 degrees Centigrade (248 degrees Fahrenheit). There are, however, several drawbacks to increasing the operating temperature of the cooling system, which include reduced lifetime of cooling system components such as the radiator core, radiator hoses and water pump seals. In addition, increasing the coolant operating pressure may actually have an adverse effect on cooling at certain critical points in the engine, particularly in systems where a significant amount of (liquid-to-vapor) phase-change cooling occurs. For example, the most efficient cooling occurs at an engine cylinder wall when coolant conditions are conducive to nucleate boiling. An increase in the operating pressure of a given system elevates the coolant boiling point and impedes nucleate boiling, thereby decreasing the heat transfer from the cylinder wall to the coolant. This may lead to occurrence of hot spots in the engine which may accelerate component fatigue, cause detonation, and excessive NOx emissions.
It has been estimated that under typical driving conditions an automotive ICE generates only about 30% of available power 90% of the time. In the remaining 10% of the time, such as when accelerating or climbing steep inclines, engine power output is higher than 30% of available power and, in some cases it may approach maximum engine output. However, periods of such high power demand are quite limited in duration.
Phase Change Materials: For the purposes of this invention, a material that changes in heat content upon undergoing a reversible solid-liquid phase transformation is defined as a phase change material (PCM). PCMs, synonymously known as latent thermal energy storage materials, are used for thermal energy storage. The absorption of the necessary quantity of energy by the solid PCM results in melting. The energy absorbed by the PCM to change phase at its characteristic melting temperature is known as the latent heat of fusion. The latent heat of fusion stored in the liquid state is released upon resolidification. Thus the PCM may absorb thermal energy from a body at a higher temperature than the PCM, until the PCM undergoes a reversible melt. A molten PCM may transfer thermal energy to a body at a lower temperature than the PCM and it may thereby undergo a reversible solidification (freeze).
Efficient PCMs have several desirable thermo-chemical properties including high latent heat of fusion, high thermal conductivity, low supercooling, and the ability to cycle thermally from solid to liquid and back to solid many times without degradation. The term “supercooling” refers to a discrepancy between the temperature at which solidification (freezing) initiates and the melting temperature of a given PCM when cooled and heated under quiescent conditions. A significant amount of PCM research is devoted to finding nucleating agents additives that will suppress supercooling. The term “additives” includes, in addition to nucleating agents, precursors of such additives which are non-detrimental to the function of the phase change materials. Considerations for selection of suitable PCMs may also include melting temperature, density, packaging, toxicity and cost.
Thermal Batteries: Proposals have been made to incorporate a thermal battery into a coolant loop of automotive ICE. Such a battery is intended to store heat during normal ICE operation and release it later to warm-up the engine and/or the passenger compartment of a vehicle during a cold engine start. The battery may store heat in latent heat of a PCM which melts as the battery is charged and solidifies as the battery releases heat. PCMs used in such batteries have a melting temperature well below the normal operating temperature of the engine cooling system. Therefore, thermal batteries of this type are not capable of absorbing or releasing latent heat of their PCM at temperatures higher than the normal operating temperature the cooling system. Hence, such batteries cannot provide overload capability to engine cooling systems.
In summary, there is a need for means and methods that would allow an engine coolant system to handle temporary increase in heat load without the need to increase the physical size of the system's components and without the need to increase system's operating temperature. Suitable means should be very compact, lightweight, and inexpensive to manufacture and integrate into ICE systems, especially in automotive vehicles.